1. Field of the Invention
The present invention relates to an apparatus for quantitative secondary ion mass spectrometry and, more particularly, to such an apparatus suitable for use in quantitatively analyzing trace amounts of light elements contained in a material.
2. Description of the Prior Art
An apparatus for quantitatively analyzing secondary ions using a mass spectrometer is known wherein ion (primary ion) beams such as argon, cesium or the like bombard a material, e.g., a metal material or a semiconductor material placed in a sealed chamber kept under vacuum so as to allow it to emit elements contained in the material as ions (secondary ions) and to quantitatively analyze the emitted secondary ions (see R. E. Honig: Advances in Mass Spectrometry, edited by A. R. West, Applied Science Publishers, Barking, 1974, p.337). In an apparatus for quantitative secondary ion mass spectrometry of this type, when trace amounts of a light element contained in a sample such as hydrogen, carbon, nitrogen, oxygen, silicon, sulfur and the like are analyzed, the interior of the sealed chamber, particularly, a surrounding portion of the sample to be analyzed placed in the sealed chamber must be kept at an ultrahigh vacuum so as to reduce background light elements which are contained in the residual gases in the vacuum.
However, in a conventional apparatus, the overall sealed chamber is evacuated simply by an ion pump or a cryopump. For this reason, the interior of the sealed chamber cannot fully be evacuated, and residual gases undesirably remain therein. In addition, contaminating gases are released from the surface of the sample, and contaminating gases can also be introduced into the sealed chamber from a primary ion source. A pressure in the sealed chamber, particularly at the surrounding portion of the sample, is increased by these gas components, and light elements as major components of these gases cannot be prevented from sticking in the surface of the sample. FIG. 1 shows a conventional apparatus provided with a cryopump. The apparatus includes a sealed chamber 1 which is provided with the cryopump 7 at its one end. The cryopump 7 is constituted by a refrigerator 7a, a cryopanel 7b, an outer cryopanel (or shield panel) 7c and a baffle 7d. The sealed chamber 1 holds therein a sample holder 4 on which a sample S is to be placed, an electrostatic lens 5 and a deflection electrode 6. A pipe is connected to the chamber 1 and provides a passageway for a primary ion beam 3 generated from a primary ion source (not shown). Only the cryopump cannot fully evacuate the chamber 1. Further, even if an ion pump 8 is provided for the chamber 1, a surrounding portion of the sample S cannot be fully evacuated. When the light elements are analyzed in such a state, a detection limit of a target light element is limited by the residual gas pressure near the sample, and light elements contained in a sample in trace amounts cannot be analyzed. For example, FIG. 2 shows an analytical result of the oxygen detection limit when the present inventors quantitatively analyzed an impurity oxygen contained in a gallium arsenide (GaAs) crystal using the conventional apparatus. As is apparent from FIG. 2, in the conventional apparatus, the detection limit of oxygen is on the order of 10.sup.16 /cm.sup.3. Since an impurity oxygen concentration in the high purity GaAs crystal is on the order of 10.sup.15 /cm.sup.3 or less, such a low impurity concentration cannot be analyzed by the conventional apparatus.